1. Field of the Invention
This invention generally relates to a liquid crystal display and, more particularly, to a pixel structure of a wide view angle liquid crystal display and to a driving method thereof.
2. Description of the Related Art
In large size liquid crystal display, as pixels are spread over a greater area, a user is impossible to look squarely at the image shown by each pixel on the display during watching. Therefore, brightness and contrast differences will be observed by the user while watching at different angles. In order to overcome this problem, wide view angle techniques have been proposed and a scheme named multi-domain vertical alignments (MVA) has been confirmed to be able to effectively improve the color washout. The capacitance coupling type (C.C. type) pixel structure is a sort of wide view angle technique. Although it is not necessary to change the driving method of a display when using this type of pixel structure, the image sticking is another issue to be considered during image display. Another wide view angle technique, named two transistors type (T.T. type) pixel structure, has solved the image sticking problem, but the number of gate lines or data lines used in this pixel structure has to be doubled such that the manufacturing cost will be increased.
To solve the problems existing in conventional pixel structures, Samsung Electronics proposed a charge-shared type pixel structure 9, including a sub-pixel 91 and a sub-pixel 92, at SID Symposium Digest 2008, as shown in FIG. 1. Through charge sharing between capacitors, the sub-pixel 92 and sub-pixel 91 of the pixel structure 9 may have different gray level voltages during operation. However, because the capacitor CS will keep at the voltage of previous frame before TFT3 is turned on, the sub-pixel 92 is difficult to accurately reach a desired voltage level through charge sharing when TFT3 is turned on. Accordingly, the gray level shown by the sub-pixel 92 for each frame will be influenced by previous frame and is different from actually desired gray level.
Please refer to FIGS. 1 and 2, FIG. 2 shows a voltage timing diagram of the capacitors in the two sub-pixels shown in FIG. 1 when the voltage of the pixel structure 9 switches from a high gray level to a middle gray level, wherein t1 is a time period that the first gate line Gn turns on the pixel structure 9 and t2 is a time period that the second gate line Gn+1 turns on a pixel structure, adjacent to the pixel structure 9, connected thereto. As shown in FIG. 2, within the time period t1, the first gate line Gn simultaneously turns on the switching transistors TFT1 and TFT2 such that the voltage VC1 of the liquid crystal capacitor CLC1 of the sub-pixel 91 and the voltage VC2 of the liquid crystal capacitor CLC2 of the sub-pixel 92 decrease to a middle gray level voltage together according to the voltage of the data line Data. Within the time period t2, the second gate line Gn+1 turns on the switching transistor TFT3; meanwhile, through the charge sharing between capacitors CLC2, CST2 and CS in the sub-pixel 92, the voltage VC2 of the liquid crystal capacitor CLC2 of the sub-pixel 92 can be different from the voltage VC1 of the liquid crystal capacitor CLC1 of the sub-pixel 91.
Please refer to FIGS. 1 and 3, FIG. 3 shows a voltage timing diagram of the capacitors in the two sub-pixels shown in FIG. 1 when the voltage of the pixel structure 9 switches from another gray level (e.g. a gray level lower than the initial gray level shown in FIG. 2) to the same middle gray level, wherein variations of the voltage VC1 of the liquid crystal capacitor CLC1 of the sub-pixel 91 and the voltage VC2 of the liquid crystal capacitor CLC2 of the sub-pixel 92 are similar to those shown in FIG. 2. The difference is that, the sharing capacitor CS has a lower voltage VCS′ before the second gate line Gn+1 turns on the switching transistor TFT3, so the voltage VC2′ of the liquid crystal capacitor CLC2 of the sub-pixel 92 shown in FIG. 3 and the voltage VC2 of the liquid crystal capacitor CLC2 shown in FIG. 2 will have different gray level voltages after the switching transistor TFT3 is turned on. That is, the gray level voltage of the sub-pixel 92 during each display period will be influenced by the gray level voltage of previous frame.
Accordingly, it is necessary to provide a pixel structure of a liquid crystal display that can more correctly control the gray level voltage of sub-pixel.